Changing the licensing terms for an application

[Hanxue Lee]

I wish to know whether I can change the licensing term of an application
from GNU GPL to BSD or vice versa.

For example, the original progam is licensed using BSD license. I found out
that some unscruplous developers are adding some new features and re-selling
it without contributing back to the source tree. I wish to avoid this, so
can I change the licensing terms from BSD to GPL? How about the other way
round? If the licensing terms could be changed, is it only the original
owner can change it?

Yours Truly,
Hanxue

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